88 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



In liis reconnaissance of 1901 Willis encountered the formation which he 

 described in the following words :— 



' On the North fork of the Flathead there are, as already stated, bluffs 

 of clay with interbedded sandstones and lignites, in which no fossils were 

 found. Details of constitution are summarized in the tabular statement 

 of formations. The materials, degree of induration, and the lignitic con- 

 dition of the carbonaceous deposits serve to indicate that they may be 

 of Miocene or Pliocene age, as are beds near Missoula, which they resemble. 

 These deposits are called lake beds because they are very distinctly and 

 evenly stratified. They consist of fine sediment such as would settle from 

 quiet water only, and they occur in a valley of such moderate width between 

 mountains of such height that no simple condition of alluvial accumulation 

 seems appropriate. It is possible that the lake was at times shallow like 

 a flooded river. It is probable that it was some time reduced to the pro- 

 portions of a river. It is certain that during considerable intervals some 

 areas were marshes; but, admitting that a lake may pass through various 

 phases of depth and extent, the term lake beds best describes these depo- 

 sits.'* 



At the Boundary line the dip is 18° to the eastward. Farther north the 

 attitude is fairly constant in all the exposures, with strike north and south and 

 dip, 40-45° east. The formation has evidently been disturbed by a strong 

 orogenic force. The date of this particular phase of mountain-building cannot 

 yet be fixed with certainty. It is pre-Glacial and post-Laramie. With some 

 probability it may be referred to a mid-Tertiary stage, during which, according 

 to Willis and Peale, crustal deformation took place in Montana. 



It would be a matter of considerable interest to know the nature of the 

 terrane underlying the lake beds. The fact that the drill at the Kintla creek 

 oil-prospect struck continuous limestone at the depth of 1,290 feet suggests 

 either that the lake beds lie directly on the Cai'boniferous or pre-Cambrian, 

 or else, that only a very small thickness of Mesozoic strata (presumably Cre- 

 taceous) intervene between the lake beds and the pre-Mesozoic formations 

 beneath the floor of the valley. This point will be considered again in connec- 

 tion with the dynamic history of the Rocky Mountains at the Forty-ninth 

 Parallel. 



POST-MIOCENE FORMATIONS OF THE GREAT PLAINS. 



For the sake of completeness Willis' brief statement of the occurrence 

 and nature of the youngest geological formations found on the plains in the 

 immediate vicinity of the Boundary belt, may be given in summary. On pages 

 328 to 330 of his paper he gives some details concerning a Pliocene or early 

 Pleistocene gravel fan to which the name ' Kennedy gravels ' has been given. 



* B. Willie, Bull. Geol. Soc. America, Vol. 13, 1902, p. 327. 



